Rain-on-snow events, floods and climate change in the Alps: Events may increase with warming up to 4°C and decrease thereafter

This paper focuses on the influence of mountain rain-on-snow (ROS) events that can on occasion trigger major floods in alpine catchments. In order to assess the evolution of these events in a recent past, and the potential changes that could be experienced in a changing climate over coming decades,we have focused on a small catchment in north-eastern Switzerland, the Sitter, well-endowed with both climate and hydrological data. Observations showthat there has been an increase in the number of rain-on-snow events since the early 1960s related to the rise in atmospheric temperatures. Results froma simple temperature-based snowmodel showthat the number of ROS events could increase by close to 50% with temperatures 2–4 °C warmer than today, before declining when temperatures go beyond 4 °C. The likelihood of more ROS events suggests that the risks of flooding in a future climate may indeed get worse before they improve.